High performance aromatic polymers feature, because of their very high glass transition temperatures and/or melting temperatures, excellent properties including an outstanding heat resistance. Aromatic polysulfones and polyetherketones are, for example, widely used in applications where their strength, resistance to harsh chemicals and to high temperatures is necessary.
Unfortunately, many natural and synthetic polymers such as the above mentioned high performance aromatic polymers are prone to light absorption and are attacked by UV radiation. As a result, they undergo oxidation, chain scission, uncontrolled radical recombination and cross-linking reactions. This phenomenon, known as UV degradation, is usually catalyzed in high heat environments in the presence of oxygen. The UV degradation of polymers can affect a material's mechanical properties, produce discoloration and fading, roughen the surface, decrease tensile strength, and reduce their overall life time performance.
A wide range of light and heat stabilizers for polymers are known and have been used alone or in various combinations to prevent or retard the kinetics of polymer degradation that is initiated by exposure to light and heat. The effectiveness of stabilizers to defend a material against UV radiation and heat depends on several factors including; the intrinsic efficacy of the stabilizer, its concentration, and its solubility in a particular polymer matrix, as well as how well it is distributed in the matrix. Intrinsic volatility of the stabilizer is also an important factor to consider when working with materials which are processed at high temperatures as it may lower the concentration of the stabilizer in a particular polymer matrix as a result of evaporation during processing and subsequent use.
Over the past century, a number of light stabilizer compounds have also been developed and commercialized as additives tailored to retard or eliminate photo-initiated oxidative processes. These additives are generally categorized into one of 4 classes: UV absorbers, excited state quenchers, radical scavengers, and peroxide decomposers. Certain derivatives of 2-hydroxybenzophenone have been known for a long time to improve the light stability of polymeric compositions. For example, U.S. Pat. No. 3,192,179 discloses their use in low melting temperature polymeric materials such as certain polyester-styrene resins and polyvinyl chloride resin.
Nearly all commercially available heat and light stabilizers are indeed well suited for blending with low melting temperature commodity polymers requiring low process temperatures (i.e. below 250° C.).
However, such commercial heat and light stabilizers are prone to thermo-oxidative decomposition or volatilization upon exposure to high temperatures (i.e. above 250° C.). Therefore, they are generally poorly suited for high performance aromatic polymers where process temperatures are substantially more intense compared to low melting temperature commodity polymers.
There exists a need, therefore, to identify and develop stabilizer compounds that are well suited for high performance aromatic polymers in that the polymer compositions made therefrom possess high temperature mechanical performance, good thermal-oxidative stability and good light stability.
The present invention provides such stabilizer compounds (SC), end-capped stabilized polymers (ESP), and methods for their preparation and use.